Competitive formation of polymer-cyclodextrin inclusion compounds

The hydrophobicity of the guest polymer and also the geometrical compatibility between guest polymer cross section and cavity diameter of the host cyclodextrin (CD) play important roles in the formation of inclusion compounds (ICs) between a mixture of one or two guest polymers with one or two different types of CDs, respectively. Specific polymer-CD interactions can be distinguished when, for example, polymer A-CD IC crystals are suspended in a solution containing polymer B, and a polymer B for polymer A exchange occurs, without CD-IC dissolution, during formation of polymer B-CD IC. When using the polymer pair poly(epsilon-caprolactone) (PCL)/poly(L-lactic acid) (PLLA), we have observed that PLLA-alpha-CD IC is completely converted to PCL-alpha-CD IC, while the reverse polymer transfer is almost completely prohibited. alpha-CD host molecules also preferentially included PCL chains from a common PCL/PLLA solution. In addition, observation of the transfer of PCL from PCL-gamma-CD IC crystals suspended in a solution containing alpha-CD to form PCL-alpha-CD IC crystals now suspended in a solution containing gamma-CD, while not observing the reverse transfer, has implications for the relative strength of polymer <----> polymer and polymer <----> CD interactions. Single and pairs of side-by-side parallel PCL chains occupy the channels of PCL-alpha-CD IC and PCL-gamma-CD IC crystals, respectively, so our observations suggest that interactions between included PCL chains, which are only possible in PCL-gamma-CD IC, do not lead to an increased stability for polymer-CD ICs.